Recently, a quantum cascade laser, hereinafter called as a “OCL”, is gathering much attention as a solid state light source that emits electromagnetic waves in mid infra-red range or in terahertz (THz) range. Having both properties of a light and a radio wave at a time, electromagnetic waves in THz range in particular feature such high resolution capability as a light and such high transmission capability as a radio wave, with milder impact on examined objects than X rays or the like. From these points, electromagnetic waves in THz range are expected to be utilized in applications for, such as, substance identification and human body scanning examination by wave transmission.
Typical emission mechanism of QCLs utilizes a semiconductor superlattice, which uses potentials only by electrons' conduction bands having an alternating structure of wells and barriers, for example. In short, stimulated emissions by way of intersubband transition are provoked among the subbands that are created in the semiconductor superlattice. The emission mechanism in QCLs is significantly differ from one in conventional semiconductor lasers in this respect, in which stimulated emissions of electromagnetic wave are provoked by way of recombination of an electron and a hole across an energy gap between a conduction band and a valence band. Specifically, QCLs use a potential in a semiconductor superlattice with wells and barriers, and, by applying a voltage thereto, the potential is inclined along thickness direction of the semiconductor superlattice while having well/barrier undulating patterns. The inclined patterned potential is then used to provoke stimulated emissions by electrons in a multi stage manner, or in a cascade scheme. To fabricate a semiconductor superlattice that is capable of such transitions, it is necessary to conduct “band engineering” in which thicknesses of well and barrier layers are precisely designed in consideration of the inclination due to the electric field. In QCLs, repeated use of conduction carriers, or electrons, enables a carrier recycling.
In QCLs, it is possible to cause lasing operation with a wavelength that has no relationship at all with an energy gap of material for the semiconductor superlattice, and on top of that, the lasing wavelength can be tuned through designing process of the semiconductor superlattice. For these reasons, QCLs have been made for emission of electromagnetic waves emitters in THz range, for which wavelength no solid state light source was devised. Such QCLs in the THz range, or hereinafter called “THz-QCLs”, have been classified into several types in view of schemes to make population inversion for lasing. One example of THz-QCLs is called “bound-to-continuum” type, in which the electromagnetic wave is emitted by electrons that make transition from an isolated level to a miniband that forms a continuum band. A THz-QCL of this type is disclosed in Non-Patent Document 1, in which the THz-QCL operates at an oscillation frequency of 3.65 GHz, where the population inversion is created by relaxing electrons in a lower lasing level by way of electron-electron scattering within the miniband. It should be noted that THz-QCLs of bound-to-continuum type may have high voltage efficiency; however, such THz-QCLs need complicated designing and suffer from significant adverse effects of scattering by LO-phonon.
Another type of THz-QCLs is disclosed in Patent Document 1 (U.S. Pat. No. 6,829,269). The THz-QCL disclosed in Patent Document 1 uses three electronic levels per one stage of stimulated emission. That is, one additional electronic level is adopted to form population inversion between an upper lasing level and a lower lasing level that need the population inversion for lasing operation. The THz-QCL in Patent Document 1 adopts an upper lasing level (|3>, referred to as “level L3” in the present specification), a lower lasing level (|2>, “level L2”), both of which are related to the lasing operation, and another level (|1>, “level L1”) located below the level L2. The level L1 operates to depopulate electrons from the level L2 by using longitudinal optical phonon (“LO-phonon”), (see for example, FIG. 3 in Patent Document 1). Such a three-level scheme is hereinafter called “LO-phonon assist type”.
Patent Document 1 discloses a THz-QCL of the LO-phonon assist type that has a semiconductor superlattice provided with repeating structures in its active region for stimulated emissions, where each structure, hereinafter called simply as “unit structure,” is configured to realize each stage of stimulated emission. The unit structure includes an emission region and an injection region. Energy potentials, or a band, in the unit structure have been engineered in such a way that the emission region improves the emission efficiency and that the injection region helps formation of the population inversion. In the active region of LO-phonon assist type THz-QCLs, well layers are made of GaAs, whereas barrier layers are made of AlxGa1-xAs to implement such design.